Salient 8.5.4 is Now Compatible with Woocommerce 3.3.1 . Kindly update the Theme if you have updated Woocommerce . 

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Woocommerce Payment Gateway Not Showing ( BrainTree / Amazon )

Add this into the Custom CSS box located in your Salient Options panel (Make Sure there are no red cross in the Box) :

/* show any Iframe added by Payment Gateway as theme has inline style that hides the iframes by default */
.woocommerce-checkout iframe {
    opacity: 1 !important;
}
.woocommerce .payment_methods iframe {
    opacity: 1 !important;
}
.wc-braintree-hosted-field iframe {
    opacity: 1!important;
}